I just mixed up (as close to as I could) exactly 1 gallon of regular Instant Ocean salt water using the instructions that are online. Ideally, I would have checked the volume of my one-half cup measuring device by using a lab-grade graduated cylinder, but I didn't want to go in to the lab today.
The best way I could think of to get exactly 1 gallon of water at home is to measure by weight. One gallon of fresh pure water at 70 F weighs in at 3785g. (I know - mixing metric and imperial units drives me nuts too, just don't have the right tools at home to do it all metric). My mixing container weighs in at 351g - so one gallon of water in my container comes in at 4136g, and my temp was approximately 72 degrees (71.8 according to my digital temp thermometer that I have checked for accuracy using a lab grade device).
Unfortunately, I had to be much more imprecise in my measuring out of the salt mix. I used a stainless steel kitchen one-half cup measuring cup. I added over a cup of salt, and using the back of a kitchen butter knife leveled it off. I was careful not to compact the salt, just to level it off.
I then put a powerhead into the 2 gallon bucket I used to weigh out my water added the salt carefully so that there was no water loss to splashing, and then covered to prevent any small evaporative loss. I let the water mix up for one hour. At the end of that time, the mixed IO NSW was clear with no visible residue on the bottom.
I calibrated my refractometer (just to be safe!). The salinity reading was slightly above 1.023 - or approximately 31ppt (maybe a bit above the line - hart to tell on that side of the scale). The calcium reading I got (again, using best practices, with an aquaforest Ca kit - which I get incredibly consistent results with) was approximately 390ppm, and ALK came in at 9.3dkh. I do not test for Mg, but I should order a Salifert kit for that.
Results could vary because of various types of error - and there are a literal plethora of potential causes here. The volumetric measurement of water or salt mix is imprecise using non-lab grade measurement devices (I have to say here that I did check my postal scale against a balance in the lab and it was off by less than a gram using a 1kg standard). The salt has been compacted. The salt has recrystallized due to humidity - this could effect how much of the salt mix can fit into a measuring device. I would think humidity would cause partial disolution of the chemicals causing a loss of space between crystal grains - this would reduce air space in the mix and (I would think - if anything) cause the resulting salinity to be higher since there would be less air in the salt mix. Best would be by weight, but I could find no mix by weight instructions on IO's website.
I had to mix up a bit of Instant Ocean NSW today because I am cutting the thick base off of a Lobo, and will need some water to replace what I use. This was a fun little exercise, and I think this is about as good a quantification as can be reasonably done using careful measurements in a non-lab environment with non-lab measuring tools. Ideally, I'd repeat 3 or more times and measure each parameter, then do the mean. But I'm not so inclined right now.
Anyhow.
